When to Tell Someone You Hate a Movie/Book/Game/Show They Love
Truly, what are you expecting to accomplish? Will your day be brightened by someone else hating something they used to love?
If someone says, "I love this thing!" and you hate it, don't feel compelled to reply that "actually, it sucked."
I'm not saying to keep it to yourself when you dislike something, I'm just saying don't reply directly to expressions of love with expressions of hate.
The only exception I can think of is if someone says they love something that's deeply problematic. If it's something that's actually pretty racist, then explain this as concisely as possible and without suggesting the person is a bad person for liking it in the first place.
But if it's someone liking a Star Wars or Lord of the Rings thing that you didn't like due to some story element that didn't land for you, resist the urge to try and make that person hate the thing that brings them joy.